Fixes OAuth 2 Strategy.

OAuth2 providers require the original redrect_uri to be included in the request
when getting an access token. This was previously including extra parameters on that uri
and getting 40x errors from various providers.

oa-oauth/lib/omniauth/strategies/oauth2.rb

@@ -48,6 +48,10 @@ def client
         ::OAuth2::Client.new(client_id, client_secret, client_options.merge(options[:client_options] || {}))
       end
 
+      def callback_url
+        full_host + callback_path
+      end
+
       protected
 
       def request_phase